Process for eliminating stain and photographic elements therefor



United States Patent 3,486,896 PROCESS FOR ELIMINATING STAIN AND PHOTOGRAPHIC ELEMENTS THEREFOR Charles Gerald Geary, Fair-haven, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware N0 Drawing. Filed Sept. 27, 1966, Ser. No. 582,234 Int. Cl. G03c 1/76, 1/48 US. Cl. 9667 7 Claims ABSTRACT OF THE DISCLOSURE Photographic film elements having a colloid-silver halide emulsion layer and a separate anti-stain layer of a chemically modified gelatin having reversible colloid properties, a low isoelectric point of 3.0-5.0, and containing extralinear carboxylic acid groups, e.g., carboxymethylated, phthalated, and maleyl-substituted gelatin. The developed film does not embody dichroic stain even when sufiicient sulfide is present to cause dichroic stain with normal gelatin layers of higher isoeletric points.

This invention relates to photographic film elements having a colloid silver halide emulsion layer and an antistain layer and to a process for eliminating dichroic stain in photographic elements having such an emulsion layer.

It is known in the art of photography that dichroic stain on the surface or throughout a silver halide emulsion layer results when sulfide is introduced into the developer solution. The most common causes of stain-forming sulfide are hydrogen sulfide in the atmosphere, bacteria in aqueous developer solutions which form sodium sulfide by attacking the sulfite in used aqueous developer solutions andthe formation from sodium thiosulfate in aqueous fixing solutions, e.g., led to the developer solution via film clips, etc. Obviously, dichroic stain can be avoided by maintaining fresh processing solutions but, this is an expensive and only temporary expedient. Stain may also be reduced by using bactericides to prevent formation of sulfides; by eliminating sulfide by precipitation with heavy metals; or by overcoating the emulsion to resist the physical deposition of either sulfide or silver from the developer onto the silver halide grains. The problems of sulfide stain are fully discussed in Henn and Crabtree, PSA Journal, 13, 752-758 (1947).

It has been found, in accordance with the present invention, that dichroic acid stain can be eliminated or markedly reduced to an acceptable minimum by applying to the surface of the water-permeable colloid silver halide emulsion layer of a photographic film element a thin antistain layer of a chemically modified gelatin having reversible colloid properties, a low isoelectric point of about pH 3.0 to about 5.0 (preferably 3.5 to 4.5), and containing extralinear carboxylic acid groups.

Novel photographic elements of the invention comprise, in order:

(1) an organic polymer film base.

(2) a water-permeable organic colloid layer having dispersed therethrough radiation-sensitive silver halide grains and (3) An antistain layer of a chemically modified gelatin having reversible colloid properties, a low isoelectric point of about pH 3.0 to about pH 5.0 (preferably 3.5 to 4.5), and containing extralinear carboxylic acid groups.

Useful supports include cellulosic esters, e.g., cellulose triacetate, cellulose acetate/butyrate; superpolymers, e.g.,

polyvinyl chloride, polyvinyl acetals, e.g., formals, acetals; polystrene; polyamides, e.g., polyhexamethylene adlpamide, and polyesters, e.g., polyethylene terephthalate,'

polyethlene terephthalate/isophthalate, esters formed by 'ice condensing terephthalatic acid and dimethyl terephthalate with propylene glycol, diethylene glycol, tetramethylene glycol or cyclohexane-1,4-dimethanol (hexahydro-p-xylene alcohol). The vinylidene chloride copolymer coated oriented polyester films of Alles et al., US. Patent 2,627,- 088, and Alles, U.S. Patent 2,779,684, are especially suitable.

The preferred photographic films of this invention have a polyethylene terephthalate base and on one or both surfaces a vinylidene chloride/acrylic ester/itaconic acid layer, then a gelatino-silver halide emulsion layer of the X-ray type and sensitive in the blue region of the spectrum, and on said layer carboxymethylated gelatin wherein the carboxymethylated gelatin constitutes 1% to 3.5%, by weight, of the coating solution. The latter layer may contain ingredients conventional in antiabrasion layers, e.g., colloidal silica; gelatin-hardening agents such as chrome alum, dimethylolurea, formaldehyde, oxaldehyde and gultaraldehyde and surfactants of the anionic cationic, and non-ionic type, e.g., saponin, sodium alkylsulfates of 12-18 carbon atoms, N- and C-cetylbetaine, etc.

The antistain layer in general is coated to a weight of 6-12 mg. per sq. dm., preferably approximately 10 mg./ dmF- When the antistain layer of an X-ray-sensitive film element of the invention, for example, is dried, the film can be exposed by a conventional X-ray machine and then developed, fixed and Washed in the usual manner. The developed filmwill be found not to embody dichroic stain, even when sufiicient sulfide is present to cause dichroic stain with similar outer layers of normal gelatin of higher isoelectric points.

The colloid silver halide emulsion layers may contain in addition to the light sensitive silver halide, e.g., silver iodobromide, silver chlorides, silver-chlorobromide, etc., various sensitizing dyes and other materials which are common in emulsion layers. Such additional materials include fog-inhibiting agents,, emulsion hardeners, emulsion preservatives, etc.

The chemically modified gelatins useful in accordance with the invention can be prepared by processes such as those described in US. 2,525,753 and US. 3,184,312. Suitable processes include reacting at near room temperature and in an anhydrous diluent, the free amine groups of the gelatin with an .anhydride or an alpha-halo fatty acid until all free amine groups have been converted to an amide linkage or substituted with a CH COOH group. For instance, when preparing carboxy-methylated gelatin, the amine groups of the gelatin are reacted with bromoacetic acid which replaces one of the hydrogens on the amine group with the (CH COOH) group and liberates HBr. Modified gelatins produced in this manner have isoelectric points of 4.4 to 4.5 or less. Phthalated gelatins with an isoelectric point of 5.0 are also quite useful in accordance with the present invention.

While gelatin, and mixtures of gelatin with covering power enhancing agents, e.g., dextran, dextrin, hydrolyzed glycogens, water-soluble pectic saccharides, laminarins, mannans and beta-hydroxyalkyl ethers of polyvinyl alcohol are preferred binding agents for the silver halide grains, natural and snytheti-c water-permeable protective colloids can be used, e.g., albumin, casein, agar-agar, alginic acid, and alginates, polyvinyl alcohols, polyvinyl acetals and cellulose derivatives.

The abrasion overcoated photosensitive elements were prepared and tested according to the following procedure. The results obtained for the various modified gelatins are summarized at the end of the examples.

EXAMPLE I A gelatino-silver iodobromide, blue-sensitive, emulsion of the X-ray type was made, digested and prepared for coating in the conventional manner. The emulsion contained 1.2 mole percent of silver iodide, 98.8 mole percent silver bromide, and had a gelatin-dextrain binder as described in Jennings, US. 3,063,038. The ratio of binder to silver halide was 1.1:1. The emulsion was coated to approximately 100 mg. of AgBr(equivalent) /d. on a polyethylene terephthalate support bearing in vinylidene chloride copolymer layer as disclosed in Example IV of Alles, U.S. 2,779,864, The film was divided into five equal parts with one of the parts serving as a control. Antistain overcoatings containing a standard gelatin and each of the modified gelatins having a low isoelectric point were then prepared as follows:

Component A: G. Gelatin or modified gelatin 200 Distilled water 6520 Component B contained the conventional abrasion coating additives, e.g., hardeners, coating aids, etc.

Component A was digested for 15 minutes at 130 F. After digestion, component B was added to the mixture.

Five overcoatings were prepared as above but each time, one of the following gelatins was used in component A Gelatin: Isoelectric point (1) Standard (control) 5.2 (2) Carboxymethylated 4.5 (3) Phthalated 4.4 (4) Phthalated 5.0 (5) Maleylsubstituted 4.5

Each of the abrasion coatings was coated over the X-ray-sensitive emulsion to an approximate coating weight of mg./dm. After being coated and dried, each sample was flashed to a medium density, X-ray exposure at 70 kvp. The exposed elements were developed in a 68 F. metol-hydroquinone, conventional, X-ray developer to which 5 p.p.m. sodium sulfide was added to simulate developer contaminated by use. The developed samples were then immersed for 3 minutes in a conventional 68 F. aqueous X-ray fixing solution containing ammonium thiosulfate. After fixing, the film was Washed in water, and dried.

The films were examined for stain, The control element, overcoated with unmodified gelatin showed appreciable mirror-like deposits when viewed by reflected light, and orange, red, and purple deposits when viewed by transmitted light. The other films were examined for stain in a similar manner and were found to have no stain, or considerably less stain, than the standard. The following is a As can be seen in Table I, as overcoating containing a modified gelatin having a low isoelectric point, preferably 4.4 to 4.5 or less, eliminates or lessens the amount of dichroic stain on developed X-ray emulsion.

EXAMPLE II Example I was repeated except that a carboxymethylated gelatin was the only modified gelatin used. An aqueous developer solution Was contaminated by use and produced excessive stain on an X-ray film using standard gelatin in the antistain layer. When the carboxymethylated gelatin was used with the same developer and processed as in Example I, no stain was present on the sample.

This invention provides a non-staining photographic film having an important economical advantage in that it is not necesesary to use fresh processing solutions.

Likewise, cross-contamination of developer and fixer as a source of stain is no longer a problem since the non-staining structure of the films of this invention essentially eliminate dichroic stain. Elimination of such stain corrects a known deficiency of prior art films and makes the reading of X-ray film less susceptible to error due to stains on the developed film.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved photographic element comprising, in order:

(1) an organic polymer film base,

(2) a water-permeable, organic colloid layer having dispersed therethrough radiation-sensitive silver halide grains, and on layer (2) (3) a separate antistain layer having a coating weight of 612 mg. per square decimeter of a chemically modified gelatin having reversible colloid properties, a low isoelectric point of about pH 3.0 to about pH 5.0 and containing extralinear carboxylic acid groups selected from the group consisting of carboxymethylated, phthalated and maleyl-substituted gelatin.

2. An element according to claim 1 wherein said organic colloid is gelatin and said silver halide grains are sensitive to X-ray radiation and to blue light rays.

3. An element according to claim 1 wherein said modified gelatin contains phthalyl groups.

4. An element according to claim 1 wherein said film base is a polyethylene terephthalate film having a thin anchoring sublayer composed of a vinylidene chloride/ acrylic ester/itaconic acid copolymer.

5. The process of improving the antistain characteristics of a photographic film element which comprises applying to the surface of the water-permeable colloid silver halide emulsion layer a separate, thin antistain layer having a coating weight of 612 mg. per square decimeter of a chemically modified gelatin having reversible colloid properties, a low isoelectric point of about pH 3.0 to about pH 5.0 and containing extralinear carboxylic acid groups selected from the group consisting of carboxymethylated, phthalated and maleyl-substituted gelatin.

6. A process according to claim 5 wherein said colloid is gelatin.

7. A process according to claim 5 wherein said modified gelatin contains phthalyl groups.

References Cited UNITED STATES PATENTS 2,956,880 10/1960 Gates et al. 9694 NORMAN G. TORCHIN, Primary Examiner R. J. LYON, Assistant Examiner Us. C1. X.R. 96-76; ass- 5 

